When /dev/console is a disconnected /dev/ttyS0, TCGETS on /dev/console results in EIO (Even if TIOCCONS is called on a PTY)

Hi

I am resending as I realized I sent in Rich Text instead of Plain Text. I am sorry if any of you got this duplicate email

Background:-------------------------------------------------------------------- This issue becomes evident on VT-less kernels. As when there is no /dev/tty0 device, the console defaults to being /dev/ttyS0 instead. Although this can also be replicated if booting a standard kernel with 'console=ttyS0' and ensure nothing is plugged into /dev/ttyS0.

This issue prevents systemd from logging to the console.
systemd when logging to /dev/console, long story short it calls isatty() on /dev/console, and when /dev/console is actually /dev/ttyS0, and nothing is connected to /dev/ttyS0, isatty() fails on /dev/console due to an input/output error, causing systemd to not log the console output, because it rejects /dev/console as not being a terminal.

This is noticed on a VT-less system with Plymouth. Plymouth calls the TIOCCONS ioctl on a pty device it requests, to redirect console output, and in newer versions, it displays the console logs on its own without the assistance of a VT.

This part of it works, Plymouth is able to 'see' what gets written to /dev/console, log output from processes that write to /dev/console directly (for example 'echo hi > /dev/console") do appear in plymouth's /var/log/boot.log, it is just that systemd is not writing to /dev/console because isatty() fails to report /dev/console as a tty device.

The alternate fix in for systemds https://github.com/systemd/systemd/pull/33690
is believed to be that when TIOCCONS is called on a PTY, or another terminal device, that trying to call TCGETS on /dev/console should no longer result in an error.

Replicating the issue:---------------------------------------------------------

This program replicates it:
-------------------------------------------------------------------------------
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>

int main(void)

{
        int fd;

        if (getuid() != 0) {
                printf("Must be root\n");
                return 1;
        }

        fd = open ("/dev/console", O_RDONLY);
        if (!isatty(fd)) {
                printf("err on /dev/console: %s\n", strerror(errno));
        }
        return 0;
}

-------------------------------------------------------------------------------

When the kernel console is /dev/ttyS0 and /dev/ttySO has no device connected, it prints "err on /dev/console: Input/output error"

When I strace it, the relevant line is:
ioctl(3</dev/console<char 5:1>>, TCGETS, 0x7f...) = -1 EIO (Input/output error)

I have not been able to come up with a proposed fix

Thanks